A pseudodipeptide is a dipeptide in which the amide linkage has been substituted with a thioether linkage. We have made several of these compounds in order to determine to what extent the molecular structure of these thioethers mimics that of a normal peptide. We are incorporating one of these derivatives into a tetrapeptide, and this tetrapeptide (AlaPhe-pseudo-Phe Ala) will be submitted for x-ray analysis. In addition, the same pseudodipeptide [S,S(-)-Phe-pseudo-Phe] has been incorporated into a pentapeptide which is related to the C-terminus of EGF (epidermal growth factor). This pentapeptide will then be incorporated into a larger EGF fragment either chemically or enzymatically, and the biological activity of the resulting material will be assayed.